This invention relates in general to the handling of articles and, in particular, to improvements in apparatus for the conveying and handling of various materials by means of air flow. The invention particularly relates to an improved air lock system for use in the pneumatic conveyance of various bulk materials with particular reference being had to the pneumatic unloading and conveyance of bulk cargo (e.g. fish) from ships wherein any damage to the product (including bruising, crushing or slicing) is of foremost concern and must be avoided if at all possible.
A typical prior art vacuum conveyance system employs an air separator chamber wherein material entering with the air flow through an elongated conveying duct can settle out under the influence of gravity/momentum forces onto a sloped or conical bottom surface. The collected material must be extracted through some form of air lock either on a continuous or batch basis. To maintain the negative air pressure necessary for conveyance of the material through the inlet conveying duct, the air leakage back through the material exit must be minimal. Several different types of apparatus are commonly employed to achieve this effect.
In one common form of prior art device, a rotary valve is employed having a plurality of radially extending vanes which come into close contact with the walls of the valve chamber. As such valve rotates, the material moves downwardly into the spaces between the vanes and is thence carried around by the rotary valve and discharged through a bottom exit. Another type of valve employs a rigid door which is hinged to one side of the material discharge duct. This door is periodically opened to discharge the material which has collected at the bottom of the discharge duct. The disadvantage with both of these systems is that they require control means responsive to the level of the material which is built up coupled with powered actuators. The failure of the actuating systems can cause the material to back up to the extent where damage is caused to certain components of the system including the air mover (e.g. suction fan). Moreover, when such arrangements are used in the conveyance of relatively large objects (such as fish) the sealing edges of the valves either tend to jam open or cause unacceptable crushing damage to a relatively large proportion of the articles being conveyed during normal operation.
Another form of system employs a head tube which is immersed in a bath of water thereby forming what is known as a "water trap seal". One form of such apparatus is illustrated in European Published application No. 80/100611.5 in the name of the assignee of the present invention. The advantage of this arrangement is that it does not tend to damage certain fairly delicate articles, such as fish, and moreover provides continuous material discharge onto the output conveyor. The system has several disadvantages however. It cannot be used where the materials must be kept dry. The system has a large daily consumption of fresh water. An even greater problem is the environmental problem raised by the daily disposal of contaminated water. Buoyant materials, such as ice, cause blockage problems or even require a very long head tube. The system is not practical for use with high negative pressures greater than 30 inches water gauge due to the large quantity of water drawn into the head tube and the resulting increase in overall height.
Another form of air lock valve incorporates the use of a heavy flexible rubber sleeve which is connected to the lower end of the discharge or head tube. Under the influence of the negative pressures in the separator chamber, the walls of the rubber tube collapse inwardly toward one another thus forming an air seal. Experience has shown that for a natural gravity discharge to occur, a relatively large material head is required and this material as collected discharges intermittently only after such relatively large head of material has built up. This requires an impractically large conveyor or fluming system to accept the intermittent and relatively large volumes of material discharged. In the case where negative pressures greater than about 10 inches water gauge are involved or where the overall head tube height must be restricted, material height or level controls must be employed which function to periodically cause a reduction in negative air pressure thereby to permit discharge of the material. This causes an interruption of the conveying operation since the lowering of the negative air pressure reduces air velocity through the inlet conveying duct to the point where material cannot be transported.